Dear John
Since there were lots of doubts from our readers on the LED emergency light, I have written a detailed letter to two of our readers. Of course few modifications are required, which I have indicated on the drawings and on calculations how I arrived at it. I thought of enclosing it to you so that it can be kept in some library of our site for any ones reference. For each circuit we can have a similar detailed theory, may not be with all calculation, just general operation theory of each part of the circuit. You may think of it. As a first feed back I am sending this. I have a feeling that most of the people would have made the mistake in base emitter of BD140, the lead out are against normal convention.

To understand the above circuit in a better way, it can be divided into two parts.

1. LED lamp circuit
2. The Battery charger circuit

LED Lamp circuit

1. All are white hi bright LEDs rated for 3Volt @ 25mA
2. The total current requirement is 12 X 25 = 300mA
3. This current has to flow through T2 – BD140 PNP transistor
4. The minimum current gain (hfe) of this transistor @ 500mA is 50
5. Hence the base current Ib requirement is Ic / hfe, 300 / 50 = 6mA
6. Base emitter drop of T2 at 500mA is 0.77 volt
7. With the fully charged battery at 6.9volt terminal voltage (for cycle operation use) the voltage available across the new bias resistance is (6.9 – 0.77)
8. Hence the bias resistance is = 6.13 / 6 = 1000ohms
9. As the battery drains the final terminal voltage will be 5.4volt
10. The bias resistance will be (5.4 – 0.77) / 6 = 770 ohms Hence a 680 ohms was preferred for bias resistance with drained battery also it will give enough brightness.
11. The very important information about BD140 is, as you view the pins, metal portion of the transistor facing down left is emitter centre collector and right is base. Most of the constructors make this mistake, relying on the convention that left base and right emitter. If you have made this mistake please correct it.

Once this portion is checked for reliable operation we will proceed to charger portion.

The Battery charger circuit

1. The battery requires a full terminal voltage of 6.9V at this point charger should cut off.
2. That is the voltage across the chain ZD1, R2 and T1 be should be 6.9 volt
3. T1 be voltage of 0.7 volt plus drop across R2 and zener voltage should be 6.9V
4. T1 be current = Ic / hfe
5. Ic is 1.25 / 180 = 7mA
6. Ibe = Ic / hfe of T1 i.e = 7 / 70 = 100uA
7. Drop across R2 =1.2 X .1 mA = 0.12volt
8. Hence Zener voltage = 6.9 – (0.7 + 0.12) = 6.08 the near by preferred zener voltage is 6.2 volt
9. Say the battery voltage at full charge will be 7 volt with 6.2 volt zener diode
10. To calculate R16 value for charging at 1 /10 th of the rated current of the battery 4.5AH / 10 = 450mAH
11. Transformer 9volt AC the voltage across C1 will be 9 X 1.414 = 12.6 volt
12. The drop across LM317 at 450mA current for good regulation is 3volt
13. The drop across protective diode D5 is 0.7 volt.
14. The voltage available at cathode of D5 is 12.6 – (3+0.7) = 8.9volt
15. The battery after fair discharge will be at 6 volt
16. Hence R16 = (8.9 – 7) / 0.45 = 6 ohms
17. The nearby standard value for operation is 5 ohms.
18. At the end point of battery 5.4 volt the maximum charging current can be of (8.9 – 5.4) / 5 = 0.7 amps well within the higher charging limit of the battery.
19. With this circuit over night the battery will get charged fully.
20. Over charging is taken care and protected by T1

Hope with the above guide line you can make your light work successfully.

More Modification!!!

Automatic LED Emergency Light with Under Voltage Cut Off Protection:

Dear Readers,

Mr. Seetharaman has further modified this LED Emergency Light with an under voltage cut-off protection to protect battery from deep discharge. Once the battery terminal voltage falls below 5.7 volts the LEDs will be switched off. Take a look at the modified circuit shown below.

To understand the above circuit in a better way, it can be divided into two parts.

LED lamp circuit

The Battery charger circuit

LED Lamp circuit

All are white hi bright LEDs rated for 3Volt @ 25mA

The total current requirement is 12 X 25 = 300mA

This current has to flow through T2 – BD140 PNP transistor

The minimum current gain (hfe) of this transistor @ 500mA is 50

Hence the base current Ib requirement is Ic / hfe, 300 / 50 = 6mA

Base emitter drop of T2 at 500mA is 0.77 volt

With the fully charged battery at 6.9volt terminal voltage (for cycle operation use) the voltage available across the new bias resistance is (6.9 – 0.77)

Hence the bias resistance is = 6.13 / 6 = 1000ohms

As the battery drains the final terminal voltage will be 5.4volt

The bias resistance will be (5.4 – 0.77) / 6 = 770 ohms Hence a 680 ohms was preferred for bias resistance with drained battery also it will give enough brightness.

The very important information about BD140 is, as you view the pins, metal portion of the transistor facing down left is emitter centre collector and right is base. Most of the constructors make this mistake, relying on the convention that left base and right emitter. If you have made this mistake please correct it.

Once this portion is checked for reliable operation we will proceed to charger portion.

The battery requires a full terminal voltage of 6.9V at this point charger should cut off.

That is the voltage across the chain ZD1, R2 and T1 be should be 6.9 volt

T1 be voltage of 0.7 volt plus drop across R2 and zener voltage should be 6.9V

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Comments

Subhankar Roy

February 17, 2015

I use this circuit diagram,but i see at the end of bridge rectifer 11.76volt,but at the output of the LM317T shows 7.04 volt and after the D5 diode 6.74 volt(when R1 is 180 ohm & R2 is 2100 ohm)…is it enough for charging 6 volt 5ah battery upto full charge?????????

and how can i change the output voltage upto 7.2 volt??????…. plzz reply

good day sir. we plan to use 12v/5Ah battery instead of 6v/4.5Ah, 7809 or other 78xx linear regulator and 15 volts transformer to power up some sensor. Also we tried to remove the BD140 so that the sensors are always on. Could you pls help us designing our circuit? What values or components should we change? thanks in advance.

thank you for your contribution. however this circuit is not functioning well. i have wired the connection exactly as mentioned in the circuit diagram. the problem is that the green led (charging/charging stop indicator) always glow and battery not charging beyond 6.40 volt. i am using 2 amps transformer but 0 current passes after 6.40 volts. according to you it should reach 6.9 and cut off. but green led does not turn off.

Hi,
very interesting subjects and projects,keep it up.We find almost all circuits invariably has a transformer but designing such transformers are not provided. can you forward me with such reference or the data sheet for 10Va-100/200Va design.
bashyam

When the battery s charging the IC output voltage will be 6.9 or more depends on the current flow to battery. once the charge is full the terminal voltage will reach 6.9volt or more the T1 transistor will conduct to bypass the reference current through R1 180 ohms. the current through it is reference voltage between pin no 2 & 1 of the IC which 1.25 volt and the resistance is 180 ohms hence the current through R1 is 7mA. this is the current T1 to bypass hence in the next step what is the T1 base current required 7 ma / hfe of T1 which is 70 etc etc.

hii. i ve just build this circuit with the absent of 4.7ohm near diode 1n4007 … and i need 7.47v at the output diode to cutoff voltage(led on the emitter and collector of bc548 to turn off) .. could we make it more low voltage needed to cutoff and where i m wrong?is the resistor 4.7 ohm missing so the circuit goes wrong?

Hi first disconnect over voltage protection circuit ZD1 R2 and T1. Now adjust VR1 for the required output voltage without connecting battery. The resistance 4.7 ohms is the charging current limiting resistance kindly do not delete. Now connect the battery and check the current flow to battery by measuring the voltage drop across 4.7 ohms. If you want higher charging current reduce the value to 2.2 ohms

Modify the current version with new implementation like
1.Touch dimmer circuit,
2.Battery over charging protection,
3.60 LED’s setup,
4.Battery low indicator with buzzer.
Sir,please consider my suggestion and upload the circuit for the same.

Hello sir, i have succeeded in making automatic emergency light using 12 LED’s but unsuccessful while using 50 LED’s, i kindly request you to help me out while making automatic emergency light using 50 LED’s. Please upload a circuit description of automatic emergency light using 50 LED’s.

I’ve succeeded with this circuit, but I still need one more modification on it. It is how to embed a night-activated function so that the LEDs are not glowing at the absence of mains power in the day light. Can you give me a brief explanation or a schematic of combining these circuits? thanks.

i have been working on this circuit since two weeks and facing only one problem again and again,

1. the charging circuit is not working properly, as LM317 is not giving regulated voltage, if i disconnect the circuit right after the LM317, it shows a potential difference of 8~9v DC between gnd and output pin of regulator(not stable). if i remove the Resistor R1 and then measure the voltage across ZD1 it gives me 6.2V DC. furthermore, i think in my circuit T1 is not switching with R1(180ohm) i replaced the resistor with a variable resistance and then tried to adjust it with circuit “ON”
at an instance when it reached only 10ohm circuit led’s started to glow fully. at that time i measured the voltage across ZD1 and adjusted with VR1 at 6.9v DC (without battery connected).
but after some seconds i felt smell and then VR1 gave a flash and gone bad.
Please guide me should i change any resistor value or put exactly the same components?
secondly, we cannot get the transistor of same hfe from market do it have any effect on the circuit or resistor?
Kindly Help me out from this scenario.
Thanks and Regards,

I try to make this under voltage cutoff circuit. But it don’t work.
Try to replace 5.1v to 5.6v zener, BC547 to BC337, 560R resistor to 1k…etc…still does not work.

With extra under voltage cut-off protection circuit, when I plugging the power source, the light will not turn off, when battery discharge down to 5.1v, lights not cut-off. I think this extra circuit will never work.

Hi Andy it is tried practically. with higher gain pnp transistor for BD140 similar to S8550 I ahd a problem of light glowing low with mains on hence R15 was reduced to 330 ohms. otherwise it is working fine at my home.

I have wired the “Automatic LED Emergency Light with Under Voltage Cut Off Protection” circuit .I used 15v transformer.I removed the led which indicates charging. but when the value of VR1 is kept at the highest i only get a voltage drop of 6.5v between gnd. if I use 6.5 volts to charge this battery, will battery ever reach 6.9V which is higher than reacharging voltage?I found that the battery has never been fully charged, furthermore the maximum output voltage is only 6.5v (battery disconnected).I tried to replace the D1 with the new ones many times but it still showed the same thing. I even replaced the T1 BC548 but nothing changed.what should i do?

For protecting the battery constant voltage is important plus the maximum current. current is taken care by IC charecterestic itself will limit to 1.5A max. the voltage by VR1 which sets the the output voltage to 6.9volt. the zener is only a backup protection in the event of potentiometer failure over a period of time.

Hi sir, i need help. the led indicator wont light. it only lights when i remove the battery from the circuit. one more thing i dont think that my battery is charging cause i only get 5.8v in the battery terminal without the battery.

HI Zild you kindly check the 317 output voltage with LED indication and also without it. without LED if it becomes OK, replace LED with high efficiency 3mm red LED which requires only 2 to 3mA for brightness.

Hi Siri 6volt is the battery terminal potential difference.(the voltage available for your use). mAH is ampere hour rating of the battery. in simpler words 4.5 AH 20 hours rating means, If you discharge at a current rating of 225mA the battery will sully the load for 20hours. For easy under standing, it is the product of the current the battery can supply in Amps for so many hours. If you draw 450mA from a fully charged battery, it will supply this current for 10hrs approximately. please go through the following link.http://www.dcbattery.com/faq.html

To calculate R16 value for charging at 1/10th of the rated current of the battery 7AH/10=0.7AH. Because voltage reference of LM317 is Vout-Vadj=1.25v, R16=(1.25+0.7)/0.7=2.78R, choose 2.7ohm 5w resistor.

For safety discharge battery under 10v, choose ZD2=10v zener 1N4740A.

Depend what type of Led, u can use formula to calcutale resistor limit: R=(Vbattery – Vled)/Aled, example if ur led working with 5v/50mA: 12v-5v/0.05=140ohm.

Dear Sir,
Can this circuit be modified for 20-24W, using 12V-7.5AH battery.
what modifications are required when we use 3W or 5W power leds.
I need it for my farm at my village place, as we have 14 hrs of power cut daily.

Dear Brother, i first disconnect ZD1 and set the required voltage correctly 6.999v through POT as desired. then i reconnect the Zener ZD1 and desired voltage remain the same. But i am facing LOW CURRENT FLOW issue, when i omitt the diode D5 the current flows as desired but when i connect D5 the current the reduces to only 20mA and at that time the voltage of the battery is 6.2v, i aslo changed D5 (i.e 1N4007) but all in vain. At such stage the current flow should be atleast 100mA which is not the case. Please sir, make me correct i am making a mistake somewhere in the circuit…..Thanks….Regard

Respected ,Seetharaman Sir,
I need a good circuit witch will fit any Regular Inverter from outside as a device,and this will working as a hybrid solar inverter, will function as a junction box of the solar pannel unit for charing the existing batery of that inverter and if not charging from solar properly then convert the charging line to the ac mains for full charge of that battery.
If this possible,then send my mail,Thanks.Rajib

i have been working on this circuit since two weeks and facing only one problem again and again,

1. the charging circuit is not working properly, as LM317 is not giving regulated voltage, if i disconnect the circuit right after the LM317, it shows a potential difference of 8~9v DC between gnd and output pin of regulator(not stable). if i remove the Resistor R1 and then measure the voltage across ZD1 it gives me 6.2V DC. furthermore, i think in my circuit T1 is not switching with R1(180ohm) i replaced the resistor with a variable resistance and then tried to adjust it with circuit “ON”
at an instance when it reached only 10ohm circuit led’s started to glow fully. at that time i measured the voltage across ZD1 and adjusted with VR1 at 6.9v DC (without battery connected).
but after some seconds i felt smell and then VR1 gave a flash and gone bad.
Please guide me should i change any resistor value or put exactly the same components?
secondly, we cannot get the transistor of same hfe from market do it have any effect on the circuit or resistor?
Kindly Help me out from this scenario.
Thanks and Regards,

Firstly, i would like to extend my sincere appreciation for the wonderful work that you and this site are doing.

I want to manufacture a few complex circuit for use at home and would like to know how i can contact you personally. I have a flow diagram and know how the system should function but i do not know how to bring it into practical use since i am not strong in electronics. I would really appreciate a mail from you in this regard. Thank you in advance

respected sir,
AFTER SHOWERING OF TOO MUCH COMMENTS NEED ITS TIME TO MODIFIED
YOUR CIRCUIT AGAIN.BCZ ITS DIFFICULT TO READ 400 COMMENTS.ALSO I WOULD LIKE TO NOTE THAT EVEN AFTER 4 VOLT IN BATTERY STILL DEEP DISCHARGE NOT WORKING.ALSO BATTERY TERMINAL VOLTAGE NEVER REACH ATLEAST 6.9V.

Sir,
I have one doubt about the battery charger part of the circuit. Is it possible to get 400mA at 8V considering the diode D5 forward resistance of few hundred ohms added to circuit?(I’m getting only 40mA at the output at 8V)
At my place the supply voltage is about 170 to 190 V during most of the times, so do i need to change the transformer?

i have been working on this circuit since two weeks and facing only one problem again and again,

1. the charging circuit is not working properly, as LM317 is not giving regulated voltage, if i disconnect the circuit right after the LM317, it shows a potential difference of 8~9v DC between gnd and output pin of regulator(not stable). if i remove the Resistor R1 and then measure the voltage across ZD1 it gives me 6.2V DC. furthermore, i think in my circuit T1 is not switching with R1(180ohm) i replaced the resistor with a variable resistance and then tried to adjust it with circuit “ON”
at an instance when it reached only 10ohm circuit led’s started to glow fully. at that time i measured the voltage across ZD1 and adjusted with VR1 at 6.9v DC (without battery connected).
but after some seconds i felt smell and then VR1 gave a flash and gone bad.
Please guide me should i change any resistor value or put exactly the same components?
secondly, we cannot get the transistor of same hfe from market do it have any effect on the circuit or resistor?
Kindly Help me out from this scenario.
Thanks and Regards,

I found that the battery has never been fully charged, furthermore the maximum output voltage is only 6.4v (battery disconnected). When I took out the T1 BC548, the output voltage was 9v. As you said, in the previous comments, that it’s due to the defected D1 zener 6.2v 0.5w. I tried to replace the D1 with the new ones many times but it still showed the same thing. I even replaced the T1 BC548 but nothing changed.
I also need to know how many sets (BD140+12 LEDs) can be supplied by this circuit? Can I use 5 or 6 sets?

Thanks for the circuit. I’ve tried it and it worked perfectly. But there is something I don’t understand, It’s how to adjust the VR1. The red LED glows all the time and I’m just worried that the battery is over charged. Would you please give me a brief explanation on how to correctly adjust the VR1 so that I am sure that the battery is not charged all the time but stops when it’s already fully charged. Thanks.

Can we use PWM circuit using 555 to extend the working time?
So the LED wont be on all the time, but due to persistence of vision we can see continuous light.How much improvement can we expect with that?

Mr.Seetharaman, your circuit is really good. Can you help me in modifying the circuit, so that we can add two more led lights in series to all the leds. This will result in high power light output. Pls reply to mail with the circuit. Thanks.

hello, sir. I have a 12v 12ah Battery and i want to use “12v 10w rounded Fluorescent tube”…( like car’s ones )
i need 2 link 3 pieces of this Fluorescent tube.
so, i want 2 know the new modifications for that circuit which are related to :
1-the suitable Transformer (volt and ampere)
2-ZDs (volts and watts)
3-BD140
4-LM317
5-the other components.
6-for how many hours will the lighting keep on in full-charging case ?

hello, sir. I have a 12v 12ah Battery…what if i want to use a “12v 10w rounded fluorescent (like cars’ ones)”
i will link about 3 pieces of this fluorescent type..
what modifications should i perform on
1-Transformer
2-ZDs
3-BD140
4-LM317
5-other components.
and for how many hours will it keep ON lighting in full charging case?
Regards,

Respected Sir,
I want to add one more feature to your circuit, the leds should be controlled with an ir remote,how can I make such a modification to your circuit ? If possible please refer some websites from where I can perform these modifications.
Thank you.

Respected sir,
I was tried so many times sir,but battery discharging circuit not working sir i used 5.1 v znr but below 4 volt also led continuously glowing sir…i checked the component everything correct.but deep discharging not function sir.automatically switching and every thing is good sir next what can i do please explain me sir my mail id Krishna198023@gmail.com Help me sir.

im currently using ur circuit here but cant seem to find a way to make it work, i modified it with my own specification… heres what i modified recently…

-instead of 12 led i used 24 leds
-replaced the battery to 12v with 7.2ah ( supposed to be 7.5ah but cant seem to find one in our place)

sir i tried using ur original circuit in multisim v11 but the LED indicator for charging doesnt work.. i also tried to put a voltmeter in parallel to the battery to see if its charging but seems to me that its not

with ur current skills maybe u could put me in the right direction, thank you very much for ur help

the voltage across the cathode of ZD1 and gnd is only 6.4V. when i removed the battery and the zener ZD1 and made the voltage at the charging poing of the battery(which was the junction of ZD1 and 4.7 ohm ) as 7v. the voltage drops when i connect the zener and also the ZD1 is not conducting.

Hi Shankar You disconnect ZD1 and battery. now measuring the output voltage going to battery for charging. set it for 6.9 volt DC now connect the battery check the voltage drop across R16 and calculate the current flowing with a flat battery it should be 500mA and partially charged battery it can be between 50 and 250mA. If this current is not available your LM317 pin connections are wrong or your transformer is not capable of 9 volt RMS at 500mA. check throughly. this is a proven circuit design.

i also checked the voltage across zener when it is reverse biased , it is 6.2v. and the zener diode also works when connected to a 9v battery with 1k in series and in reverse bias , giving a voltage 0f 6.2v across it. i also checked the R1 and pot resistance. i have been trying this for many days now. please help.

i did wire the circuit exactly as in figure. i find out that the zener is not concucting and the voltage over the zener changes when the value of VR1 is changed(when battery not connected).i have not connected the battery.and the potential difference with respect to the gnd at the cathode of zener, zd1 is max 0f 6.9 . when i remove the transistor T1 the output of LM317 is raised till 9v. I tried changing zener diode(6.2v 0.5w)many times but it does not get corrected.i also tried changing BC548, and LM317 . still i have the same problem. i find out that zd1 is not conducting in reverse bias when the battery is not connected.PLEASE HELP…PLEASE.. 🙁

i have wired the “Automatic LED Emergency Light with Under Voltage Cut Off Protection” circuit . the led which indicates charging has been glowing for more than 20 hours.. i dont know till what value the lm317’s output is to be adjusted. but when the value of VR1 is kept at the highest i only get a voltage drop of 6.64v betewwn gnd. what should i do?

I need to design a display using 5×7 matrix LED with certain light output specifications for commercial purpose. Can i contact you directly at your email id for assistance, if yes, give me your email id.

sir i have build a charging circuit that uses a BD 140 transistor to charge a 4.8 NiCd battery. Can i please know the operation of this transistor and also how can i calculate the taken for my battery to be fully charged and the time it will last when used. Thanks.

i have tried this circuit with 2 1watt led in series and kept all the same but i have a problem that when the circuit is powered by battery it feedback some of voltage to regulator in my case nearly 5 volt and charging led lights. what should i am doing wrong any suggestion to me

hello mr.seeetharaman..this circuit design is very helpful for me..could u pls help me in making the same circuit but with more number of LEDs which is bright enough like a CFL lamp..is that possible? if so do i need to change any other component as well for this change? do pls help..
thank you very much..

Hi Philips. Use 3nos AA rechargeable cells with limiting resistance per LED. or use a driver capable of 3.2 volts for an input of 2.2 to 2,64 volts. use a simple capacitance-rectifier-resistance charger circuit direct from mains for 1/12 of rated current of the cell.

hi the emergency lamp is working great- i now plan to make a rechargeable LED head lamp- it doesn’t need to be automatic- i intend to use a 3 volt hi bright LED and 2 AA size rechargeable cells to keep it light- it just need voltage regulation and over charge cut off- can you please help me sir?

Hi Pillai CFL requires around 110 volts at high frequency. If you want to convert it into an LED emergency light, except the battery and the switch remove all the other electronics available inside. replace them with the new assembled LED PCB. I prefer you to try the simple one appeared earlier use the following link.http://www.circuitstoday.com/automatic-led-emergency-light#simple

I have a working emergency cfl lamp.Can I replace the CFL lamp with say 10-20 leds? What are the required circuit modification for the same.From where should I tap the supply for the LEDs.Also how much voltage does a CFL require for working??

Hi use 2 in series and 3 of then in parallel it will be 900mA drain on the battery you will get around 5hrs of non stop lighting from a fully charged battery. (4500mAH / 900mA = 5H) Similarly you can calculate for other loads

Hi Philipif you use 2 in series and 3 of then in parallel it will be 900mA drain on the battery you will get around 5hrs of non stop lighting from a fully charged battery. (4500mAH / 900mA = 5H) Similarly you can calculate for other loads

This relates to Fridge Door Alarm circuit and need your support as there might be no response at message sent at url of said circuit.

I have constructed above project on zero board and chekced all connections but used 22uf as 10uf was not available with me. circuit produces a very little sound and not a BEEP! no affect observed when LDR is brought to dark or light, pl help in finding error.

i also want to know, why a high value R3 (2.2M) and R4 (1M) are used, is it the reason??
thanks.

I have a working emergency cfl lamp.Can I replace the CFL lamp with say 10-20 leds? What are the required circuit modification for the same.From where should I tap the supply for the LEDs.Also how much voltage does the CFL require for working??

Hi John if you are using 12 volt transformer. the DC will be 17volt battery requirement is 7volt a drop of 10 volt at 1amp is 10watts dissipation on LM317. you have to use a good heat sink. regarding 4.7 ohms resistance at 1amp it will generate 5 watts approximate hence you have to raise the rating to 10 watts. first set the output voltage without connecting to battery for 6.9 to 7.2volt then connect battery things should become alright.

sir, i have problem in my emergency light charging power supply its only give 3.85 volts to battery what the problem sir i made 3 emergency light same circuit diagram but problem appeared in power supply its only give 3 to4 volts only i am thankful to you sir please reply regard to?

Hi Bilal By disconnecting T1 and charge indicating LED check regulator output voltage it should be 7volt if not check regulator IC pin connection it is not similar to 78XX series ICs. please refer to IC manual and connect , it should become alright. still if you have problem check the wiring fully. it is a proven circuit it should work ok.

Hi,
I made the circuit and its working fine but,the transformer is getting hot.ie;i cannot hold my finger on transformer not more than 5 seconds.Is it the fault with transformer or the circuit?Please help

i have many question regarding this led light
Q1:why we use zenor diode?
Q2:what is the use of bc548?
Q3:can we use diode bridge in this circuit?
Q4:how much leds will be connected in this circuit?
i m wating for your ans thanks………

Q1. Ans: Zener diode is a active semiconductor device which has a heavily doped junction. It is always connected in reverse bias mode. It’s used to maintain a fixed voltage across, or in simple words, the zener diode is used to regulate voltage.

Q2. Ans: BC 548 is a npn transistor. It’s used in the circuit to regulate the charging of the battery along with the zener diode.

Q3. Ans: Yes you can use a i Amp bridge, or you can make a bridge using four IN4007 diodes.The scenario changes in case of implementation of center-taped transformer.

Q4. Ans: built another array of led’s with limiting resistors and use a driver transistor BD 140 !!!

Sir,thank you for the circuit.But the problem for me is that when the main supply is off,the led’s still glow with the charging led still on.Help me out sir.I have to show this circuit tomorrow as my project

Hi, charging LED can’t be on when mains is off. This is because, if you see the circuit, you will find that there is a diode D5 (IN4007) which is forward biased when mains is on, and acts as reverse biased when on battery. i guess your diode might be leaky. Please check it using an analog or digital multimeter. And to control the automated illumination use the switch !!!

Sir,thank you for the circuit.But the problem for me is that when the main supply is off,the led’s still glow with the charging led still on.Help me out sir.I have to show this circuit tomorrow as my project

Hi Bashir it looks like your LM317 is a faulty one, replace it with a reputed make from standard supplier. if you feel your present circuit is ok in all respects except the faint glow in charging Diode add a Zener diode of 3.3 volts 1/2 watt in series with the LED which should solve your prolem.

Dear sir when main power supply is OFF, and the circuit is connected to the battery the CHARGING LED still glows. but when i disconnect i the wire that is connected from T2 base to the D1 (of rectifier) the CHARGING LED becomes OFF but as well the main LEDs ARRAY also become OFF. i am using transistor 8550 for T3, also T3 becomes hot. plz help in correcting my mistake, so that CHARGING LED does not glow and LEDs array remains ON when circuit is connected to battery and main power supply is OFF. Please help me sir, my circuit is almost final just facing this problem……..plz help. plz reply……….

dear sir, the ON/OFF switch as shown schematic is creating the problem. when the switch is ON then when main supply if absent and circuit is connected to the battery the CHARGIND LED GLOWS which should not be. But when i OFF the switch then the CHARGING LED does not glow and the circuit operates perfectly. but this switch has be kept ON as whenever main power supply become unavailabe the Lighting LEDs should then go ON. so where i have done mistake that by keeping the ON/OFF switch ON the current flows in such away that the CHARGING LED glows back from the battery even main power supply is off. plz reply in detail…

dear sir, my ZD5 is not getting reverse biased and hence the current flows from the battery, in a result the CHARGING LED (RED) does not go off even in absence of main supply. please help in this regard.

dear sir, i am really thankful to you for your kindness, brilliant support and knowledge sharing attitude especially for people like me who are beginner to electronics.
sir, my issue of current flow to the battery has resolved by correcting pin configurations of lm317. but sir if you dont mind, i want to ask one more question. i.e. every thing is working perfectly but sir when main power supply is off, the CHARGING LED INDICATOR still glow, while it should be off as at that time main supply is off. sir, kindly reply why it is behaving like this…thanks once again dear sir for your tremendous support.

dear sir, i have removed T1, and then checked the flow of current to the battery and issuse is still there, the current flowing is still at 1mA. ( i have used correctly 4.7 ohm sir, and have thoroughly checked each and every component as per required specification). sir i have also removed ZD1, but it also did not solve my current issue. i once again installled the component on breadboard, (all new components) but the issue is still there even in the new circuit on breadboard, sir. i am completely confused sir at this stage what to do becasue i have checked all the possibilities………plz sir reply in detail and help me out of this problem…….

sir i have discconnected T1, but the current does not raised to its desired position, also i have changed my transformer from 12v @1A to correct specified one i.e 9v @500mA. but the issue of the low current flow to the battery and voltage drop still remains there. (i have the C6V2 5t written on ZD1..plz check it). Also sir when the main source is disconnected the current and voltage that flows to the load is perfectly all right. but i am amazed that why there is low flow of current to the battery instead the battery is drained properly as you said. sir, i am also doubtfull if the T1, T2, and T3 are soldered other way round, but may be its not true as the current from the battery flows perfectly fine to the load. please explain in detail so that i can overcome the issue…

you mean sir if i disconnect T1, and the current comes back to the desired position, then i should change the ZD1. (my ZD1 is C6V2 5t written on it) or should i change the T1. please explain a little bit more in detail. so that i can act accordingly.
Also sir i am really thanful to quick response and you are really helping alot of people.

dear sir, i drain the battery from 6.6v to 6.0v for almost 45 mintues more than you said, but the current issues still exisists there and the current that flows to the battery is on 3mA. do you think that its some other problem. i should i drain more the battery say to 5v etc. plz reply…

Hi Bashir please drain the battery with a load of 1amp for 30mts and try the charging current it will increase. a fullt topped up battery cannot consume any current, hence only a very small tricle current will flow.

dear sir thank alot for reply, yes sir i have installed a good heat sink the voltage has become stable at 6.9v but i am facing current issue i.e the current that flows to the battery is only 1mA and not 450mA as you mentioned. how to resolve the current issue.

Hi Bashir you have to raise the voltge output to 6.9 to 7 volt. a fully charged battery terminal voltage will raise to 6.9 to 7.2 volts. Check whether the heat sink of LM317 voltage regulator is adequate if not use a good heat sink to reduce its dissipation once the IC getting heated up the output will fall down due to it’s built in thermal protection acting, hence a very good heat sinl for LM317 is a must.

sir i am using 12v@1A transformer, i have assembled the circuit correctly and throughly checked each component. the voltage to the battery is set to 6.4v but there is a current issue. the current that flows to the battery is very limited and its only 3mA and i think the battery requires about 450mA and above. sir why current is so much less, 3mA is no thing for charging 6v 4.5Ah battery, although my battery is brand new. i have also checked it with another battery but the issue remains the same. sir plz reply me where i have done the mistake.

dear sir, i have set the voltage in open circuit without connectiong the battery to 6.9v using 4.7k pot but while monitoring the voltage on digital multimeter, the voltage slowly goes down and reached to about 6.4v to 6.5v. i mean the voltage as you have mentioned should be set to 6.9v which i did, but it is not static and doesnot remain its position but rather it slowly goes down. sir plz reply me what mistake i have done.

Dear sir, i have assembled the circuit and have over come my mistakes, but i am still facing one problem that the charging stops at 6.6v, also when i disconnect the main supply the LEDs comes on automatically but with a little delay, say a second. please comment.

dear sir, i really appreciate your brilliant efforts and your support for the readers, especially for those who are new to electronics as i am. i have assembled the circuit as mentioned but i have some problems.
1) first i have adjusted the pot 4.7k and its perfectly set to 6.9v in an open circuit but when i connect the battery and measure the current then its not 450mA while charging battery, and the current passed is about 19mA approximately. why?
2) i connected a fully charged battery from some other source with the same specification to my circuit and tried to check the volatge at T3 but it is not passing the voltage and shows zero voltage on the meter, why?

Thanks for your reply Sir. Another questions if you may, What is the purpose of BC548 transistor and how did you compute for the base resistance value 1200 ohms? Suppose I would change it to a different value, will it affect the circuit’s performance?

Hi Lan the gain spread for BD140 is given clearely in Philips component data book. Th gain curve with respect to collector currnt is also given, you can interapolate the curve. With the sample used the gain may vary due to the tolerance of the components used. I have given clearly it is the minimum guaranteed gain. in practice it may be more than that, which will not affect the performance of the circuit.

Dear Mr. Seetharaman:I am a non technical man. Your circuit “Modified Automatic LED Emergency Light using LM317″ is is a very good circuit, I like it, but charging time overnight is much,can you help me, if charging be completed within 5 hours please. email id akansari@gmail.com
I will be very thankful, if replied please.

Dear Mr. Seetharaman:I am a non technical man. Your circuit “Modified Automatic LED Emergency Light using LM317” is is a very good circuit, I like it, but charging time overnight is much,can you help me, if charging be completed within 5 hours please.

Good day Sir!I’m just wondering how did you get the hfe value of 50 @ 500 ma from the BD140 transistor. I have the specification sheet but I can’t figure out how did you come up with the mentioned values. Thanks.

Good day Mr. Seetharaman. Can you mail me the solution for your computations in the LED Lamp Circuit and Battery Charger Circuit? We made this really nice circuit as our project, and we need to justify every values measured in it. I have done some computations but I came up with different values with the one you did.

I send my warmest gratitude and appreciation in advance for your candid response.

hi sir!i am a second year engineering student.i am gonna do a mini project on the automatic low power emergency light.sir,please tell me the operation of the circuit of automatic low power modified emergency light.i am not getting a clear picture of the working.

Sir I want to ask that can I use 9V 250mA transformer because I have one of it and the casing in which I am making this light has also do not have much enough space to adjust a 500mA transformer so what should I do to make the circuit compatible with 250mA transformer.

Hi Nitin if you open circuit the zener only the stray and PCB leakages will affect but if it is in circuit it should operate normally. Have you checked R15 value it should be reduced to 330 ohms as I have explained earlier for LEDs to go off when mains is available. Kindly check your assembly step by step, there may be some mistakes, the above circuit is a proven circuit.

sir,
i need white led based brightest
emergency light use with 12v
7Ah battery and need back up
for minimum 8 hours.which one
is better constant current driver
or pulse boost led driver. please
suggest me some circuits.

so, if this is happening then under voltage protection ckt will not work, because during under voltage zener will not pass any current (open), but due to stray fields(as u said) leds will remain on, so is there any permanent solution?

Hi Nitin since T2 is a PNP transistor its base bias will from negative that is supplied by T3. (you may call it as return path of bias- T2 emitter base current flows through R17 T3 to negative.
Check by shorting base emitter leads of T3 (by a screwdriver tip)light should go off, indicating no bias is available for T2. By opening Zener ZD2 the base of T3 is floating stray field / print leakage can pass some current to T3 which in turn can bias T2 for glowing LEDs.

in ckt when transistor T3 is ON, then according to me current through R17 will be flowing away from base of T2 towards the negative terminal of battery through collector of T3, so potential across R17 will be such that it will be positive (bcoz of flow of current from right to left through R17) at base of T2 , so how will T2 (PNP) get turned ON?
pls reply

i need white led based brightest emergency light use with 12v 7Ah battery and need back up for minimum 8 hours.which one is better constant current driver or pulse boost led driver. please suggest me some circuits.

Sir, I have tried the modified circuit, my problem is the LEDs turn on while it is connected to mains. What do you think is wrong with the circuit Sir? When I tried to disconnect the bc548, r18 and zd2, it worked.

Hi Ajitkumar remove T1 and check whether it is raising. if ok then the zener diode is defective. if no check the pot value and R1 value
Hi chandru disconnect the power plug and use the on off switch provided to operate the light as a normal light

Hi Dojo for a 12 volt battery you require 13.8 volt cycle use, 14.4 volt for storage use and 15.2 volt for boost charging (to activate the battery). For 14.4 volts you can use one 12 volt zener with 3nos 4148 or 1N4007 is series with it to achieve the required protection. with battery disconnected set out put voltage at the wire connecting to battery with VR1 to 13.8 volts. (To achieve this your transformer should be rated for 12volts or higher)

Hello sir, thanks for the reply. As I wrote above, I’ve changed the circuit for 12V battery. In this case I think I have to set the pot VR1 for output of 13.5 Volts.
I need the protection part, to keep the battery safe.
These days I was thinking to do another experiment. to use a smaller voltage zener diode connected in series with a bipolar diode, like 11.5 volts (made of two zeners connected in series). Do you think that this may help on the higher tolerance zener diodes?
Thank you in advance!

Hi Dojo disconnect BC148 and set the pot VR1 till you get 7 volts at the battery end without battery connected. now connect the BC148 if the voltage falls down zener is of higher tolerance, if you feel you cannot get a better close tolerance zener delete bc148 portion fully, as this is only an additional protection towards over voltage. if you feel all these adjustments are difficult kindly use our simple emergency lamp circuithttp://www.circuitstoday.com/automatic-led-emergency-light#simple

Dear Sir, I have the same problem as Rizky has. I’ve adapted this circuit for 12V/7Ah battery and used a 12V Zener for ZD1 and a 5K potentiometer. The problem is that the output voltage to battery is maximum 12.2 Volts, and it is variable only from 1.1 to 12.2 Volts. If I disconnect the zener diode, the battery voltage increases and can be set to around 13.5V (and up to more than 20Volts. I am using a 24Volt/4Amp transformer). What do I do wrong?
Thank you!

Thank you Sir for your reply.
I’ve been using 9-volt transformer. but indeed in my town is very difficult to obtain components with a precise value. not only the transformer, but all the available components have a large tolerance value.
maybe I need trial and error until this circuit can work.

Hi Rizky i hope you are using a 9 volt transformer with the rated current capacity, if so no problem should be there. i think you are using a 6volt transformer instead of 9. The input voltage to LM317 will be 12.6 volt the drop across LM317 is 3volt hence you should get 9.6volt. temporarily disconnect ZD1 and battery measure the voltage output to battery with the pot you must be in a position to adjust 7 volt. now just connect the zener diode D1 and check the output voltage (still battery disconnected) if the voltage falls down your 6.2 volt zener is higher tolerance type actual voltage across it may be less than 6 volt. you

in other post you suggest to temporary disconnect zener and measure the voltage. i follow your suggest then i measured the voltage. at pin out lm317 raise to 8.41 volt, at cathode D5 7.82 volt, at end terminal (open circuit) 7.79 – 7.80 volt.

then i check the zener diode and it is indeed true 6.2 volt zener diode. the installation was correct, the anode is connected to the output leg of R2 (1.2k) and the cathode is connected to the output leg of R16 (4.7/5w).

i am new in electro. i confused how to adjust voltage. where i must check 6,9 volt – 7 volt? at pin out lm317 or at end of terminal (to battery)?

if you mean 6,9 – 7 volt at end terminal (open circuit without battery), would not that would open the zener diode and send bias to t1 then causes the voltage breakdown (t1 thought that the battery is full)?

I assembled your circuit, but i have problem. with 10k pot in highest resistance, the voltage after R16 (4.7 ohm/5watt) only 6.21 – 6.26 volt.
and then i checked other point, output from rectifier diode (D1-D4) 9.5 volts, voltage after c1 and at pin in lm317 9.5volt. voltage at pin out lm317 drop into 6.8 volt, voltage after D5 (Chatode) drop into 6.3 volt, ad then voltage after R16 drop into 6.21 – 6.26 volt.
what wrong with my circuit? thank you for Your help Sir.

i assembled the circuit, but the voltage output for connect to Battery only 6.26 volt.

i use 10k pot vr. the voltage out from D1-D4 9.5 volt, after C1 still 9.5 volt. at lm317 pin out the voltage 6.8 volt. and then after D5 voltage drop to 6.3 volt. after R16 (4.7 ohm/5watt) voltage drop to 6.26.

i try to charge the battery but the voltage of battery only up to 6.41 volt and not raise again.

for36 LEDs of 3volt 25mA you require 3nos BD140 LED drivers with each stage driving 12 LEDS. you require 900mA current say 1amp for 12 hrs use you require a 6 volt 12AH battery. the charger should be capable of 3amps same above circuit with ICLM338 instead of LM317.

sir, in the first time i run my project it was running normal but yesterday my project got into trouble. how can i fix it? this time even if the lamp is in a source, it still light. what should i do to fix it sir?

sir,i assembled the circuit and it works very fine. during the day time also if the supply is off, light is switched on. in order to stop this i suggest to connect an ldr. but where to connect it?and another thing is that what is the maximum number of leds can be used in the circuit?

Hi Richards you are correct it is EBC. If you are using 4.5AH battery and say your load is 50mA each chain 12chains 600mA. Hence the battery should last for 4500mAH / 600mA roughly 7hrs. To Find out the current use 2nos LEDs inseries apply 6volts DC to it check the current drawn.http://www.datasheetcatalog.org/datasheet/MicroElectronics/mXvusvr.pdf
Hi Shruti I dont understand your requirement.

Thank you sir for the help. My project was successful, although i made some few changes it still works. I will share to you my few changes later. Now i have to work for the documentation of my project.
Thanks a lot for you’re help.

no more fireworks for today 🙂 im so glad. i connected the wiper to the indicator. the circuit’s already ok. when connected to mains, the battery is charging. when unplugged, the LEDs glow..however, i still have the problems about the LEDs. 🙁 guess i’ll be workin on it when im not busy again . thanks man

Hi Richard The circuit is correct. Each loop is rated for 6 volt as the LEDs are rated for 3.0 volt at 25mA the excess 3.0 volt is dropped, by 0.6volt at BD140 and balance 2.4 volt is dropped across 100ohms series resistance.

dear sir,
thank u for earlier reply.. sir , is it possible to assemble modified version of your circuit on simple emergency PCB ? if you have PCB layout for the Modified versions of the circuit ..plz provide to me.. realy i need it.. plz make areply.. waiting for your reply.. joshnair@yahoo.com

Dear Mr. Seetharaman
I need some more detailed information about the function and operation of ZD1, T1 and R2. I appreciate if you send me any documentation you might have to my Email address:assemshawkat@hotmail.com
Thanks/Regards

i have assembled the ciruit in a general purpose pcb.. its working ..nice..realy ur circuit is usefull..sir, will you plz mail the pcb and its layouts if you have designed it.. i really need it.. hope that u will help me.

Hi Richard to isolate the problem without connecting to mains, link collector and emitter of BD140 if the LEDs are glowing normally, indicates problem around BD140. If the glow is still bad check the battery voltage and the resistance. may be you have to check the LEDs individually. Try the first step you will succeed the collector base emitter pin outs are different for BD140 from the traditional pin outs. watch.

Hi Richard if you donot want it say during day time when you are away you may not need it hence the switch is shown,
Hi Pete you can use 8550 most recent transistor for BD140 for BC148 you can use 2N2221 or 2N2222.

Hi Aseem you require 2.5 mp power pack. ou have to change the transformer to 9volt 3amp capacity. change all 1N4007 rectifiers to 1N5404. change LM317 to LM338 with proper heat sink. other components need not be changed.

Juan, 230V 50Hz is the AC power supply you get from a socket on your switch boards in your house. please take a two pin plug, cut it at one end you have two wires now, connect these two wires to your transformer primary.

Seetharaman or anybody out there, i just wonder what do you call that part of “230V 50Hz” ?? i mean what shall i tell the seller if im gonna buy that part? i already have all the parts excet this one .thanks

Hi Maegan that is charge indicator. once the charge is complete the LED will go off. With LED with 2.2K you cannot get open circuit voltage of 6.9 to 7.2 volts (without connecting the battery) hence you have to increase to 4.7K In place of BD140 you can use the recent 8550 PNP planar epitaxial transistor. You can use 5mm high bright white LEDs . Hope this will help.

(a)in the modified version (not the voltage cut-off protection), the pot to be used is 4.7K or 2.2K??
(b)and also what could be an alternative for BD140?? i can’t find one..
(c)can i use 5mm white LEDs instead of 10mm??
(d)what is the purpose of the LED near the IC and 1K resistor??

(a)in the modified version (not the voltage cut-off protection), the pot to be used is 4.7K or 2.2K?? (b)and also what could be an alternative for BD140?? i can’t find one..
(c)can i use 5mm white LEDs instead of 10mm?? (d)what is the purpose of the LED near the IC and 1K resistor??
please answer briefly.. i want to do this for our project..